1. Field of the Invention
The invention relates to a power supply device, and a lighting device and a lighting fixture using the same.
2. Description of the Related Art
In power supply devices used for various purposes including illumination, a chopper control method is generally known as a control method for taking out the required power. By using the chopper control method, the power supply devices have relatively little energy loss. Then, control circuit, which can achieve the chopper control method comparatively easily, is made as an integrated circuit and is commercially available from various manufacturers.
In the chopper control, turning on and off a current is repeated with high frequency, and desired electric power can be obtained. Then, a control for making an output voltage higher than an input voltage is called a boost chopper control, and a control for making an output voltage lower than an input voltage is called a step-down chopper control.
In regard to operating modes of the chopper control, there are a continuous mode, a critical mode and an intermittent (discontinuous) mode which are categorized according to whether a current that flows through an inductor constituting a chopper control circuit is continuous or intermittent. Each operating mode has advantages and disadvantages. The continuous mode is generally suitable for use in a device for outputting high electrical power, and high noise is generated from a switching element constituting the chopper control circuit, and an inductance value of the inductor also becomes large. As a result, the continuous mode has a feature that the inductor easily increases in size. The critical mode and the intermittent mode are suitable for use in a device for outputting low electrical power, and relatively-low noise is generated from the switching element, and the inductor can be reduced relatively in size. Furthermore, it is known that these two modes have relatively high efficiency compared with the continuous mode. In regard to a difference between the critical mode and the intermittent mode, when the same electric energy is taken out, the intermittent mode has a tendency to have a peak value of a switching current higher than the critical mode. Then, the intermittent mode has a feature that a ripple component of a switching period is generated easily in an outputted voltage waveform.
Further, there has been a power supply device which performs the step-down chopper control so that an output voltage becomes constant and operates to switch between the continuous mode and the critical mode (for example, see Japanese Patent Application Laid-Open No. 2009-240114). The power supply device operates in the continuous mode when supplying high electrical power, and operates in the critical mode when supplying low electrical power.
First, the critical mode is explained using FIGS. 10A and 10B. FIG. 10A is a waveform chart of a current flowing through a switching element. FIG. 10B is a waveform chart of an inductor current flowing through an inductor.
When the switching element is turned on, the inductor current is increased along a slope that is determined by an inductance value of the inductor, and energy is stored in the inductor. Then, after the elapse of an ON period Ton1 that is set previously in a control circuit, the switching element is turned off so that a detected supply voltage becomes a desired voltage. At this time, the energy stored in the inductor is supplied to an output side via a smoothing capacitor, and an average current Iout1 is outputted.
When all of the energy stored in the inductor is supplied, a polarity of a voltage generated in an auxiliary winding of the inductor is reversed. The control circuit detects the voltage polarity reversal generated in the auxiliary winding, and the switching element is turned on again with a timing of when the control circuit detects the voltage polarity reversal. Thereby, the power supply device operates in the critical mode.
Next, the continuous mode is explained using FIGS. 11A and 11B. FIG. 11A is a waveform chart of a current flowing through the switching element. FIG. 11B is a waveform chart of an inductor current flowing through the inductor.
When a supply power is high, an ON period of the switching element is lengthened so that a detected supply voltage becomes a constant voltage. The ON period becomes Ton2 (>Ton1). The inductor current is increased along a slope that is determined by an inductance value of the inductor, like the case where the supply power is low.
When the switching element is turned off after the elapse of the ON period Ton2, energy stored in the inductor is supplied to an output side, and an average current Iout2 is outputted. In this case, a voltage generated in an auxiliary winding of the inductor is inputted to a timer circuit. In the timer circuit, an upper limit value Toff1 of an OFF period is set. When time that has elapsed since the switching element is turned off exceeds the upper limit value Toff1, the switching element is turned on before the polarity reversal of the voltage is generated in the auxiliary winding. Thereby, the power supply device operates in the continuous mode.
That is, when the supply power is increased gradually, the OFF period of the switching element that is controlled in the critical mode is lengthened gradually. Therefore, when the time exceeds the upper limit value Toff1 set in the timer circuit, the OFF period of the switching element is fixed at the upper limit value Toff1.
Here, when the abovementioned power supply device operates in the continuous mode, the OFF period of the switching element is fixed at the upper limit value Toff1, but the ON period is not fixed. As a result, when the supply power is increased, the ON period is lengthened and then a drive period of the switching element is also lengthened. In other words, a drive frequency of the switching element is reduced. Then, the drive frequency is reduced to a zone of audibility, and therefore, there is a possibility that noise is generated or the drive frequency interferes in a frequency used in an infrared remote control.